Testing device for electrical windings and the like



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Aug. 25, 1959 H, RWEED 2,901,695

TESTING DEVICE FOR ELECTRICAL WINDINGS AND THE LIKE Filed July 2. 1954 2Sheets-Shea*l 1 Q fl l -l H 0N CASE Clossa 5y Cova? 70 ACTU/wf SOLE/vomCP1 AND 71'11/5 o/vrAc-rs Av, lf2, /wo ka.

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ATTORNEYS.

1 t time au zsezc IOOV Aug. 25, 1959 H. R. WEI-:D 2,901,695

TESTING DEVICE FOR ELECTRICAL WINDINGS AND THE LIKE Filed July 2, 1954 2Sheets-Sheet 2 FIGJ Geauga JNVENToR.

J'Hokrfa 75m/s -ATTQRNEYS.

TESTNG DEVICE FOR" ELECTRICAL WINDINGS ANDT HE LIKE Herman R. Weed,-Worthington, Ohio; assignorto Robbins Myers, Inc., Springfield, Ohio, acorporation of (Ehio Application July 2,-19541-Serial No'. 441,058-

Claims. (Cl. S24-573 This invention relates to aV testing device forelectrical windings-and the like and more particularly to an electronicdevice for applying-to a coil or winding tol be tested a surge voltage,while at the same time applying a like surge voltage to a standard coilor windingI and observing the instantaneous differences ofl thevpotentials across the two windings on a synchronized oscilloscope.

The surge voltage generator operates on the principle of repetitivedischarging of a charged capacitor into the coil or winding to betested. Thus, the initialmagnitude of the surge voltage and the energywhich must be dissipated in the coil under test are determined exactlyby the size of the capacitor and the charge on its plates.

When this surge voltage is appliedl to a winding to be tested, the timeVvariation of voltage across the coil to be tested depends upon its exactelectrical properties including the numberl and direction of turns, the'insulation of each point on' the coil'to ground'and to every other pointon the coil, the connections of the Winding parts to the comniutator, ifthe winding be an' armature, the wire size, series and parallel paths,an'dineiiect every possiblefac'- tor whichv contributes tothe electricalcharacteristics of the winding. The electrical behavior of the' coil orwinding, when subjected tothe surge voltage, indicates ina combinedresult the effect' of all these factors; and any deviationin thecoil orwinding characteristics; willproduce achangev off some sort in itselectrical`rbehavior. If the behavior; of` the winding'I cani'k be'-observed, as for example on an oscilloscope' scr'een, such-deviationswould be detectable- However, the deviations"aredificult and-oftenimpossible to detect-because of-'th'eir'smallsize as'- comparedto thenormal pattern 'of voltage-'or current variation, By the use-cf theprinciple? of'fnull` detectionfor; difference measurement whichemphasizes-deviations from tlenormal patterns and suppresses thosepattern-components which are' due to n'or'ma'lfand'desired electricalproperties,

the deviations can he" morel readily observed: When identical surgevoltages arel thus applied-toa standard coil or Winding having desiredqualities, and' toa coil-or winding under test, the instantaneousdifference ofthe potentials-across the two "windings is 'observed on asynchronized oscilloscope and-"if there is-a deviation in the electricalproperties of the coil' under test; Ia recognizable difference patternofy a particularshape will be produced.

With the foregoing considerations in mind, it is an object of thepresent invention `to lprovide a'testing device for testing variouselectrical properties of a coil or winding. lt is another object oftheinvention-to provide a device by means'of which the electrical behaviorof a coil or windingito be tested mayl be compared with the electricalbehavior'of a standard coil or winding in such a manner thata'characteristic pattern-will'appear on an oscilloscope screen;from-which pattern the characteristicsand f qualities of the coil undertest may be` determined lt is another object of the invention ytoprovide an-electronic apparatus f or applyingsudden surge voltages to astandard coil and coils under test and to applyva signal 2,590l Patentedug; 25,r 1959' 2" dependent upon-the difference betweenpotentials acrossthe two coilsto the input of anoscilloscope.

Still anotherob'ject ofthe invention involves the provision of a triggercircuit to control the'application of the surgefvoltageV above referredto andalso to trigger the sweep ofthe oscilloscope.

Still another object involves the provisionofan apparatus vas aboveoutlined which will be simple and safe in operation and inexpensive inconstructionv and maintenance.

These and other objects ofthe invention which will be pointed-outingreater detail hereinafter or which will be apparent to one skilledinthe art upon reading these specifications, are accomplished by. thatvcertain construction and arrangement of parts and by that series ofmethod steps, ofwhich an exemplary embodiment will now be described. v

n Reference is made to the drawings forming a part hereof and in which:v

Figure l is a general wiringdiagram of the apparatus.

Figure 2^ is a graph showing-voltageplotted against time in microsecondsto show the relationship between the triggering apparatus, theoscilloscope sweep and the testwcircuit Figures Sito l0 inclusive showthe appearance of the oscilloscope screen under'various conditions toindicate the difference patterns which' may be found. y

As'outlirfedabove, a supply voltagev is applied'through a rectier tubeto two dischargeV capacitors in series. Means'are'provided to'a'djustthe primary voltage of the supply transformer'solthat the capacitors maybe charged toany predeterminedpeakvoltage, say from 200 to 2,000 voltsThe' common'point between the discharge capacito'rsis grounded and fromone capacitor a discharge ypath leadsA to a standardV coilv and from theother capacitor a duplic'ate'discharge path leads tothe coil` tofbetested, and the common pointl between the two coils is connected to theinput of an oscilloscopel by means oflacapacitor dividerto ground. Aseparate trigger circuit isprovided to tiiiggertthedischarge ofthe'discharg'e capacitors as well as the'sweep of theoscilloscope'. Thetwo identical surges off voltage applied toth'e" coils are of oppositepolarity with respect to ground sof that, the twowindings being inseries', the potentialsof ytheirl common point with respect to groundmay be observed. The two' surges are simultaneously 'applied to the twocoils with a maximum potential differencev olf up' to 4,000 voltsaccomplished byl two setsof: triggered thyratrons.

The two discharge capacitors are rechargedl once each cycle from a6`0-cycle per second supply and the triggering of? the`thyratrons iscarefully synchronized with the 60- cycle source so that electricalconnection to the supply line isimpossible during the surge application.A trigger is.- also provided to excite the single shot trigger sweepeircuit ofthe oscilloscope.

Referring-.now in more detail to the drawings, the supplyz) transformerfor the discharge'capacitors is indicated at=T-. The primary voltageofthe transformer T may be adj'ustedby means of a continuously variabletransformer indicated lat T1, the transformer T1 being connected toacommercial 6'0-cycle supply. The charge on the capacitors to produce anypredetermined peak voltage may be determined by adjustment of thetransformer T1.

The secondary of the transformer T is connected to they two dischargecapacitors C1 -an'd C2 and the rectifier tube Rv all in series. With thetube R in series, the capacitors C1 and CZ-will be' charged tothemaximum value of the supply voltage across'the terminals be each timethe point a-is positive with respect to thepoint b. Tho-rectifier tubekR will not permitthe discharge capacitorsC1andC2 to dischargethrough thesource nor will it connect them across the line ab again until they havebeen discharged into the test coils. The common point of the capacitorsC1 and C2 is grounded as shown, and it is the potential of the points dand e with respect to ground that will institute the surge voltage. Itshould be noted that the point d is positive with respect to groundwhile the point e is negative with respect to ground. It is essentialthat the capacitors C1 and C2 be as nearly identical as possible.

The discharge path into the test coil or winding W1 is through the tubesTH1, TH2, TH3 and TH4. Similarly, the discharge path into the standardcoil or winding Ws is through the tubes TH5, TH5, THF, and THB. It -willbe observed that the direction of the tubes 'II-I5 through 'Il-I8 in thecircuit is opposite to that of the tubes TH1 through TI-L1, since thepolarity of voltage on the capacitor C2 is opposite to that on thecapacitor C1 as far as the discharge tubes are concerned.

The number of tubes is not important to the operation of the circuitexcept for the requirement that they must have the required forwardlbreakdown voltage of up to 2,000 volts as -a group or whatever maximumvalue is chosen, their ironization time must be short (preferably about0.5 microsecond), they must be capak ble of `withstanding repetitivecurrent surges of the value as determined by the circuit, and theirreplacement cost should be low. Various single, double and multipletubes meet these specications. The tubes here described are 2050thyratrons because they are very low in replacement cost and are muchmore readily available than other tubes. Between the bank of tubes'TI-I1 to 'Il-L1 and the test coil W5 there is provided a resistor R1 to`lirnit the current to protect the thyratrons TH1 to TH.1 from overratedsurges in the case of ground faults or other low impedance failures ofthe test coil W5.

The common point between the coils W1 and Ws is indicated at f and it isthe variation in potential of the point f with respect to ground whichis observed by the oscilloscope (not shown). Since this voltage is ofconsiderable magnitude, a capacitor divider comprising the capacitors C3and C5 is used between the point f and ground to supply the oscilloscopeinput s.

The control of the circuit as outlined to this point is of the greatestimportance. 'Ihe tubes TH1 through THB must re as nearly simultaneouslyas possible. This requires a large grid firing voltage having a verysteep wave front and having a relatively low impedance since it mustflre all eight tubes r[H1 to TH8 in parallel as far as the grids areconcerned.

The ring pulse is produced by the tubes TH9 and TH10 with theirassociated circuit components. The tubes TH9 and 'Il-110 mayconveniently be thyratrons identical to those used for tubes TH1 throughTH8. Such thyratrons make the tube replacement problem less difcult andalso produce the required low impedance and high power pulse.

The supply transformer T2 for the trigger circuit is an isolationtransformer as well as a stepJup transformer, so that the set may beoperated independently of the A.C. supply line ground. Ther firstpossible conduction path for the tube THQ is through the transformerterminals g and h, the resistor R3, the tube THQ, the resistor R4 andthe resistor R5. The point at which the tube THQ res is controlled anddetermined by two factors. In the rst place, the voltage across theterminals gh is 180 out of phase with the voltage across the terminalsba. Therefore, the tube THS cannot conduct at the same time as the tubeR. By this means, the capacitors C1 and C2 Y resistors R5 and R7 and thecapacitors C5 and C5. This phase shifting circuit is adjusted toapproximately behind gh so as to give maxim-um protection against lineconduction during discharge as discussed above and to provide a maximumtrigger pulse.

Until the tube TH9 conducts, the voltage across the resistors R4 and R5is zero. When the tube THQ res, this voltage suddenly rises toapproximately two-thirds of 310 volts or about 200 volts. Since thecircuit is substantially pure resistance, the rise time of the voltageis limited primarily by the ionization time of the tube TH9 which isabout 0.5 microsceond. This sudden rise of the potential of the point jwith respect to ground constitutes the steep wave front of voltage whichused to lire the tubes TH1 through THS. That portion of this voltageappearing across the resistor R5 is used to trigger the single shottriggered sweep of the oscilloscope. The scope must be sensitive totrigger voltages which are positive with respect to yground and it musthave suicient Vtrigger gain to start the sweep a-s the trigger voltagerises before the tubes'TH1 through THB trigger. This gives the sweepcircuit a lead of Vabout 0.25 microsecond on the discharge or, in otherwords, it provides the needed i signal delay to obtain a complete visualpicture on the screen. This situation is graphically depicted in Figure2 where the point x indicates the ring of tube TH9, the point yindicates the start of the oscilloscope sweep and the point z indicatesthe point at which the tubes TH1 through THB start to conduct. Thissignal delay may also be accomplished by the use of another tube with aseparately controlled tiring pointv thereby supplying two separatesignals for triggering the oscilloscope and the discharge tubes.

The tube THM, is used to terminate the voltage surge across the point jto ground and provide a short circuit path in parallel with resistors R4and R5 through the small resistor R3. The tiring point of the tube TH10is controlled by several factors. In the first place, its plate -voltageis derived from the voltage across the resistors R4 and R5 and ittherefore cannot tire until after the tube THQ has fired. In the secondplace, the combination of the small resistor Rs and the capacitor C7delay the building up of this plate voltage for about 0.5 miserosecond.(If this delay `is not provided, the tube is likely to re erraticallybecause of the sudden application of plate voltage unless a ratherelaborate grid circuit is used.) Further, the tiring point is controlledby applying a portion of its heater voltage to the control grid.Adjustment of the resistor R9 makes it possible to delay the tiring ofthe tube TH10 to the point that it will not conduct'at al1 so that thewidth of the pulse across the resistors R4 and R5 may be adjusted fromnear zero to 1r/2 radians.

The device as a whole is enclosed in -a box or case having a door orcover. A microswitch MS is associated with the cover or 'lid so that theswitch MS is closed when the case or box is closed. The switch MS is ina circuit of a control relay CR1. It will be observed that in thedischarge path to the coilts W1 and Ws there are the contacts K1 and K2.When the case is open and the microswitch MS is open, the relay CR1 isdeenergized and the contacts at K1 and K2 are open so that the dischargecircuit is shorted. When the case is closed, the Switch MS is closedenergizing the relay CR1 which .closes the contacts at K1 and K2. Therelay CR1 also operates the contact at K3 in the trigger circuit, thecontact at K3 being normally closed to short circuit the triggeringpulses and the contact K3 is opened 4by the relay CR1 when it isenergized as above outlined.

In parallel with the contact K3 there is a normally closed push-buttonswitch P which also shortl circuits the triggering pulse so that thetubes TH1 through THB cannot fire unless the switch P is manuallydepressed or opened.

The grids of the tubes TH1 through THS are capacity "3 coupled to thetriggering pulse with a 10,000 'ohm current limiting resistor in serieswith each. These resistors are indicated at R10. In `order to maintainthe possible maximum of 500 volts forward on each tube during andimmediately after the capacitors C1 and C2 are charged, a 45 voltnegative bias is placed in each grid circuit and a negatively phasedA.C. voltage is applied to their shield grids from their individualheater supplies. Tl1e45 volt negative bias in each case is provided by abattery B.

The balance between the size of the series 10,000 ohm resistor R10, the45 volt battery B and the 33,000 ohm resistor R11 is rather critical,since while the capacitors C1 and C2 are charging a positive gridvoltage will be developed across the resistor R11 because of thecharging of the capacitor C8. This must be opposed by the battery. Forthis time it would be preferable if the value of R11 were zero but whenthe iiring pulse is applied it is divided between the resistors R111 andR11 and it would be preferable if R10 were zero and R11 were large.

All the heater transformers are separate since they must operate withtheir secondaries at different potentials to ground and theirsecondari-es must have particular phase relations with respect to thesupply. The heater transformers are indicated at HT. The oscilloscopeused should have a minimum 4writing rate of 1 centimeter per microsecondsince the discharge transient lasts `for about 50 microseconds. Itshould `also have a single shot trigger sweep sensitive to positivepulses.

In its actual operation, the circuit above described deviates slightlyfrom its theoretical explanation. The tubes TH5 through TH8 actuallyrire slightly before the tubes TH1 through 'I1-L1. This is probablybecause of the fact that the tube TH5 is the 'only one of the tubes TH1through THB whose cathode is at a definite lix'ed potential with respectto ground. It will be observed that the cathode of the tube TH5 is tiedto the point e and because of the size of the capacitor C2 it is notaiected appreciably by other circuit conditions. All of the other tubeshave their cathodes floating and must depend upon stray capacity of thewires and transformers to keep their voltages fixed.

Thus, with the tubes TH5 1iring slightly before others, its iiringincreases the plate voltages on the tubes TH1; through THS and thecapacitor C2 therefore starts its discharge slightly before the tubesTH1 through TH1 conduct. This momentarily drives the point negative andthe oscilloscope shows a negative or downward peak. Perhaps amicrosecond later tubes TH1 through TH4 iire and bring the potential ofthe point f back toward Zero. This slight time difference results in aparticular voltage fluctuation of the point f that becomes -known as thereference pattern to the operator. Such a pattern has the advantage ofgiving a distinction between normal test results and a failure in theoperation of the circuit which would reesult in an absolute null ofzero. It is believed that this is a correct explanation because it hasbeen found that the time difference between the two surges variesslightly with the magnitude of the surge voltage and hence the morenearly xed potential of the point e. Observation has also disclosed thatthis time difierence will give different results when the test coil andstandard coil W1 and Ws are interchanged. As shown in Figure 1, the testcoil Wt receives the second or positive surge and this provides apattern which is best suited for interpretation. It is in this positionthat the wave of the applied pulse across the test coil W1, is of thebest form with a rise time of about 0.5 microsecond. As can bedetermined from a consideration of Figure 3 in which the horizontalgraduations represent about microseconds, the duration is roughly 35microseconds.

Although the pattern observed on the oscilloscope depends upon the typeof coil or winding under test and to some extent on the magnitude of thevoltage applied, a typical set of curves for an armature under test isshown in Figures 3 to '10 inclusive. 'I'hevpatterh will be observed tochange as the armature is rotated. A,Figrre 3 shows a typical normalvpicture with the kbrush on the commutator bar while Figure 4 representsa typical picture of a normal armature with the brush between bars.

The differences in the pattern for the variousfaults are easily noticedand can readily be distinguished from a normal picture by a relativelyuntrained operator and in most cases the operator can determine the typeand` location of the `fault. In Figure 5 there is shown a typicalpicture when the armature has the wrong number of turns. Figure 6 showsa typical picture of the total aph plied surge to the test Winding `asobserved across W1. Figure 7 shows a typical picture Iwith anopenconnection and no arcing while Figure 8 shows asimilar fault butwith arcing. Figure 9 shows the situation-when turns are shorted andFigure 10 shows the situation when there is a ground.

The relation between the pattern change and the location of the fault isdependent upon the typeof fault. Thus, for example, av ground is mostnoticeable when nearest the brush making cont-act with: the resistor R1and it is least noticeable at the-point f. On the other hand, a shortbetween turns' is least noticeable when near a brush since the brushshorts adjacent commutator bars during normal rotation. In other words,the picture on the screen rises and falls twice during one revolution ofthe armature from a normal indication to a short indication; Thesensitivity of the apparatus can be -adjusted by having the brushesseparated by amounts diftering :from depending somewhat on the type ofwinding under test.

While I have described the circuit and the apparatus in considerabledetail, it will be understood that certain modifications may be madewithout departing from the spirit of the invention and I therefore donot intend to limit myself otherwise than as set forth in the olairnslwhich follow.

Having now ffully described my invention, what I claim as new `anddesire to secure by Letters Patent is:

1. A testing device for electrical windings comprising a pair ofsubstantially identical discharge capacitors and a rectifier in series,a ground connection between said capacitors, means for charging saidcapacitors through said rectilier, a discharge path for one of saidcapacitors into a standard winding, a discharge path for the othercapacitor into a winding to be tested which is connected in seriesopposition with the standard winding, each o-f said paths comprising atleast one electronic tube having a control grid, the plates and cathodesof the tubes in the respective discharge paths being reversely connectedrelative to the two windings, an oscilloscope having one input terminalgrounded and having its other input terminal connected to a common pointbetween said standard winding and said winding to be tested, a triggercircuit including at least one electronic tube and associated passivecomponents producing a steep wave front voltage pulse,resistance-capacitance coupling means connecting said voltage pulse tothe said control grids to synchronize the tiring of said electronictubes in each of said paths, and means connecting said voltage pulse tothe input of the oscilloscope to synchronize its sweep circuit with thedischarge of said identical discharge capacitors.

2. A testing device according to claim 1, wherein the voltage supply tosaid trigger circuit is 180 out of phase with respect to the voltagesupply to the rectifier, whereby the discharge capacitors cannotdischarge through the discharge tubes while the rectiiier is conducting,and the 60-cycle supply voltage cannot appear across the windings undertest.

3. A testing device according to claim 1, wherein said trigger circuitis provided with a phase shifting circuit to control the exact point inthe cycle at which the discharge tubes start conduction.

.7 4. A testing device according Vto claim 1, wherein said triggercircuit includes an additional electronic tube positidned in the circuitto derive its plate current from the rist electronic tube in saidtrigger circuit so that it cannot re untilrsaid rst-vtube, -has fired,means to delay the ring of the additional tube for 4about 0.5microsecond, said additional tube, when it res, providing ya shortcircuit to ground to terminate the voltage pulse produced by the iringof said first tube.

5. A testing device according to claim 1, wherein said device isenclosed in a case having a cover, a mcroswitch associated with saidcover, a relay arranged to be energized by said microswitch when saiddoor is closed and contacts in the discharge paths to both saidwindings, said relay, when deenergized, disconnecting said contacts.

6. A testing device according to claim 1, wherein said Adevice isenclosed in a case having a cover, a microswitch associated with saidcover, a relay arranged to be energized by said microswitch when saiddoor isV closed and contacts in the discharge paths tofboth saidwindings, said relay, when deenergized, 'disconnecting said contacts,and short circuiting said trigger circuit, said relay, when energized,opening said short circuit.

`7. A testing Ydevice according to claim 1, wherein a short circuit isprovided for said trigger circuit, and manual means are provided to openand close said short circuit. f

8. A testing device according to claim l, wherein the trigger circuitcomprises two series connected resistors in series with a gas thyratron,the voltage rise in one of said resistors resulting from ionization ofsaid gas thyratron constituting a rst pulse, and the voltage rise acrossboth of said resistors resulting from ionization of said gas thyratron yconstituting a second pulse, the said first pulse being applied to Ysaidoscilloscope'to initiate the sweep thereof, and the said second pulsebeing applied to said discharge tubes to re them, the values of saidresistors and the gain of said oscilloscope being chosen so that theoscilloscope sweep commences before said thyratrons re. v l

9. A testing device according to claim 8, wherein of t the value of theresistance of said iirst'resistor, the value of -theresistanc'e of saidsecond resistor, and the value of Vthe :gain of said oscilloscope, atleast two of said values are adjustable.

10. A testing device according to claim 1, wherein the trigger pulse iscoupled to the control grids of the various discharge tubes by means ofa resistance-capacity coupling and a source of D.C. potential, so as tocontrol the firing of said discharge tubes without disturbing theirability to maintainthe required forward voltage across their terminalsprevious to iin'ng, and without causing the trigger .voltage itself toappear across the two windings nor to affect appreciably the nullreading of the oscilloscope.

References yCited in the le of this patent UNITED STATES PATENTS2,321,424 Rohats June s, 1943 2,584,680 A Doncyson Feb. 5, 1952 Rohatset al.: General Electric Review, September 1951, pages 51-55.

